This invention is related to molecular sieve catalysts and a method of making such catalysts. In particular, the invention is directed to a method of preparing a slurry containing molecular sieve using microfiltration. The invention is also directed to converting an oxygenate to a product containing olefin by contacting the oxygenate with the catalyst of the invention.
Olefins, particularly light olefins, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins. Particularly promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including synthesis gas derived from natural gas; petroleum liquids; carbonaceous materials, including coal; recycled plastics; municipal wastes; or any appropriate organic material. Because of the wide variety of relatively inexpensive sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
One way of producing olefins is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. See, for example, U.S. Pat. Nos. 5,912,393 and 5,191,141 to Barger et al. U.S. Pat. No. 4,499,327 to Kaiser, discloses making olefins from methanol using SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300xc2x0 C. and 500xc2x0 C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hrxe2x88x921.
U.S. Pat. No. 4,130,485 to Dyer et al. discloses a method of concentrating particulate solids having a particle size distribution from about 0.1 to 50 microns using a solid, porous, tubular microfilter. Wash water is added to the slurry while the slurry is concentrated by the microfilter until the desired purity of slurry is obtained. The addition of wash fluid is halted, and the slurry is further concentrated to a 11% solid content.
U.K. Patent Application 1,356,741 discloses a method of concentrating biological solids with at least two microfilters in series. The first microfilter consisted of a pore size of 0.45 xcexcm, and the second a 0.22 xcexcm pore filter.
U.S. Pat. No. 5,919,729 to Potter discloses the use of a microfilter to maximize the amount of zeolite molecular sieve less than 1 xcexcm in the concentrate. The initial catalyst slurry contains solids with an average size of 0.3 microns and an initial concentration of about 20% by weight solids. The concentrate of the final slurry is about 40% by weight solids. After repetitive washings with wash fluid, the concentrate is removed from the microfilter and dried by vacuum filtration.
U.S. Pat. No. 5,126,308 suggests that SAPO molecular sieve with an average particle diameter of which 50% are less than 1.0 xcexcm and no more than 10% are greater than 2.0 xcexcm lead to an increase in catalytic activity and selectivity. The laboratory prepared SAPO is recovered by centrifugation, washed with water, dried, and formed into pellets.
Inui et al. in Applied Catalysis, vol. 58, p. 155-163, 1990, shows that relatively small SAPO-34 particles can be prepared by what is known as a rapid crystallization method. This method produces SAPO-34 particles in the range of 0.5 to 2 xcexcm. The laboratory prepared SAPO is washed with water, recovered by centrifugation and dried.
After the molecular sieve particles are prepared, the molecular sieve particles must be separated from its preparation mixture or crystallization solution. Conventional laboratory-scale separation procedures include centrifugation and pressure filtration. However, both of these methods prove to be impractical for commercial-scale production of molecular sieve particles. A large-scale centrifugation process, because of the capital and operational costs, is economically impractical. In the case of pressure filtration, the smaller particles form a compacted filter cake on top the filter medium. The result is a significant decrease in flux rate of wash fluid across the filter cake and through the pores of the filter which leads to long processing times. Also, channels may develop in the filter cake which allows the wash fluid to pass trough the filter cake without contacting most of the molecular sieve particles. As a result, the molecular sieve is inadequately washed, and contaminants from the preparation mixture are incorporated into the catalyst.
The formation of the compacted filter cake also leads to very high pressure drops across the filter medium, which may result in failure of the filtering medium. Most pressure filters are designed to withstand a maximum pressure drop of about 75 psi. The pressure drop across a bed of solids is proportional to the mass flow of the filtrate through the filter, filtrate viscosity (thus, hot water is often used to reduce viscosity), cake thickness, and cake resistance. Cake resistance is inversely proportional to the square of the effective particle diameter, and proportional to the porosity of the cake. As an example, a pressure drop across a bed of 0.3 micron diameter solids will be at least 16 times that of a pressure drop across a bed of 1.2 microns diameter solids, due to smaller particle size, assuming all other properties are equal. Further, since these smaller sized particles are more compressible, the void volume (related to bed porosity) also decreases, resulting in even more increase in pressure drop. Accordingly, conventional filtration processes becomes very difficult because of these large pressure drops across beds of small particles.
Novel methanol-to-olefin (MTO) catalysts are needed which exhibit a high ethylene and propylene selectivity, an increase resistance to coking, or an increase in resistance to attrition. Catalysts with relatively small, average particle size molecular sieve could provide significant steps in one or all three of these areas of catalyst development. However, present methods of isolating commercial-scale quantities of these smaller molecular sieve particles from their preparative solutions, such as by centrifugation or pressure filtration, is either too costly and/or very inefficient. Methods to effectively recover small, molecular sieve particles, and a method of incorporating them into catalyst are needed.
This invention is directed to a molecular sieve catalyst wherein the molecular sieve is washed and concentrated as a slurry from a preparation mixture using a microfiltration process. The permeate from the microfiltration process has a conductivity from 50 xcexcmho/cm to 5000 xcexcmho/cm. The catalyst contains molecular sieves selected from aluminophosphates, metal-aluminophosphates, silicoaluminophosphates, metal-silicoaluminophosphates, and mixtures thereof. Preferably, the catalyst comprises molecular sieves comprising SAPO-18, SAPO-34, SAPO-35, SAPO-44, SAPO-47, ALPO-5, ALPO-11, ALPO-18, ALPO-34, ALPO-36, ALPO-37, ALPO-46, metal containing forms of each thereof, or mixtures thereof in the amount from 10% to 60% by weight based on the weight of calcined catalyst. The molecular sieve catalyst also contains a binder, preferably silica, silica-alumina, or alumina, present in the amount from 5% to 20% by weight based on the weight of uncalcined catalyst, and optionally a matrix material, preferably at least one clay, more preferably kaolin, present in an amount from 30% to 90% by weight based on the weight of calcined catalyst.
The invention is also directed to a process for making a catalyst. The process includes preparing a molecular sieve slurry containing molecular sieve and at least one fluid. The molecular sieve is mixed with a binder, and optionally a matrix material to from a catalyst slurry. The catalyst slurry is then directed to a forming unit, preferably a spray dryer, to produce the catalyst. The catalyst slurry preferably has a total solid content from 30% to 50% by weight.
The process of preparing the molecular sieve slurry includes concentrating the molecular sieve from a preparation mixture with a microfilter; washing the molecular sieve and any of the remaining preparation mixture with a wash fluid; and concentrating the molecular sieve from the wash fluid and any remaining preparation mixture with the microfilter. The process may also include concentrating a permeate with a nanofilter, the permeate obtained from concentrating the molecular sieve, and returning at least a portion of the concentrated permeate to a process stream used in the preparation of the molecular sieve. The microfilter pressure drop across the porous walls of the microfilter is preferably from 10 psi to 80 psi, more preferably from 15 psi to 50 psi. The temperature of the molecular sieve slurry is preferably maintained at a temperature from 10xc2x0 C. to 90xc2x0 C., more preferably from 30xc2x0 C. to 60xc2x0 C.
In some cases it is not necessary to concentrate the molecular sieve from the preparation mixture prior to adding the wash fluid. Instead, the wash fluid is added to the molecular sieve and preparation mixture before the molecular sieve is concentrated by the microfilter. In other cases, the molecular sieve may be concentrated from the preparation mixture prior to adding the wash fluid by using conventional filtration techniques, or using a microfilter with a pore size greater than 10 microns.
The invention is also directed to a method of making ethylene and propylene by contacting the molecular sieve catalyst of the invention with an oxygenate under conditions to convert the oxygenate. The process of making the catalyst includes preparing a molecular sieve slurry containing molecular sieve and a fluid, mixing a binder, and optionally a matrix material with the molecular sieve slurry to form a catalyst slurry, and directing the catalyst slurry to a forming unit.